linux-kernel - Re: aio is unlikely

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Message-Id: <20070509151831.f5956b66.akpm@linux-foundation.org>
Date:	Wed, 9 May 2007 15:18:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	suparna@...ibm.com, Ingo Molnar <mingo@...e.hu>
Subject: Re: aio is unlikely

On Wed, 09 May 2007 18:06:58 -0400
Jeff Garzik <jeff@...zik.org> wrote:

> > -#define in_aio() !is_sync_wait(current->io_wait)
> > +#define in_aio() (unlikely(!is_sync_wait(current->io_wait)))
> 
> Please revert.  Workload-dependent "likelihood" should not cause 
> programmers to add such markers.
> 
> This is a common misunderstanding about unlikely() and likely().  The 
> branch prediction used for each assumes 99% unlikely or 99% likely, 
> which is not true at all for workload-dependent code.
> 
> Even if only 1% of Linux users use AIO, for that 1%, the 'unlikely' 
> marker causes repeated branch mispredictions.
> 
> likely() and unlikely() should be used for cases where code is 
> likely/unlikely for EVERYBODY.

a) disagree with the above

b) if in_aio() ever returns true we do

	printk(KERN_ERR "%s(%s:%d) called in async context!\n",
		__FUNCTION__, __FILE__, __LINE__);

   so I sure hope it's unlikely for all workloads.
	
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